The evolution of newborn cometary ions that interact self-consistently with the solar wind are studied with one-dimensional hybrid computer simulations. Cometary ions are injected at a constant rate and in a constant direction; the associated free energy leads to growing electromagnetic fluctuations which scatter these ions. At relatively low fluctuating field amplitudes, this scattering leads to shell-like velocity distributions that subtend approximately 4&pgr; in solid angle, but have relatively small spread in speed. Shell-like distributions are favored when the ion injection rate is comparatively slow (i.e. thin shells should be observed at relatively large distances from the cometary nucleus), when the injected ion mass is relatively light (i.e. protons develop a thinner shell than oxygen ions), and when the energy density of the fluctuating magnetic fields exhibits linear, rather than exponential, temporal growth.